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Surge Stoppers Ease MIL-STD-1275D Compliance

A military vehicle is a tough environment for electronics, where the potential for damaging power supply fluctuations is high. U.S. Department of Defense MILSTD-1275D sets down the requirements for electronics when powered from a 28V supply, ensuring that electronics survive in the field.

MIL-STD-1275D compliance can be achieved by brute force, shunting high energy levels to ground using bulky passive components. This method does not guarantee power delivery to downstream electronics and can require replacing damaged protection components when they do their job. A more compelling solution is to use high voltage surge stoppers such as the LTC4366 and LT4363, which use series MOSFETs to limit the output voltage when faced with input voltage spikes and surges.

A surge stopper reference design for MIL-STD-1275D is available as Linear Technology demonstration circuit DC2150A-C. This board limits its output voltage to 44V when faced with input voltages as high as 250V, while providing 4A of current to the output in all circumstances except the ±7V ripple test, when the available current is reduced to 2.8A. In most circumstances, satisfying MIL-STD-1275D is as simple as placing this circuit in front of a 44V tolerant device. A certification report is available at www.analog.com/dc2150a-c.

MIL-STD-1275D Requirements

MIL-STD-1275D defines a variety of supply variances, from steady-state operation to starting disturbances, spikes, surges, and ripple, and lays down requirements for each of these conditions in three separate “modes of operation”:

In generator-only mode, the steady-state supply voltage is between 23V and 33V. In the simplified diagram in Figure 1, the LT4363 in combination with sense resistor RSENSE limits the maximum DC current to 4A minimum/5A typical. This protects the system from faults that occur at the output and prevents blown fuses at the input.

In Figure 1, the 250V spike condition is handled by MOSFET M1, rated to withstand over 300V from drain to source. During the –250V spike, diode D1 is reversebiased, blocking the spike from M2 and the output. (The LTC4366 surge stopper withstands reverse voltages and the –250V spike without additional protection.)

Surges

Surges are transients that last longer than 1ms. Figure 3 shows the limitations for generator-only mode. The recommended test in MIL-STD-1275D specifies that five 100V pulses of 50ms duration should be applied at the system input with a 1s repeat time. The envelope of the surge condition shown in Figure 3 is more difficult to satisfy, as it does not return to 40V for a full 500ms. The solution shown satisfies both conditions.

During the input surge, M1’s source is regulated to 66V by the LTC4366, while M2’s source (and the output) is regulated to 44V by the LT4363. Compared to using a single MOSFET, this reduces the power that must be dissipated in the individual MOSFETs and increases power available at the output.

Ripple

Ripple refers to 50Hz to 200kHz oscillations of the supply voltage about its steady-state DC voltage. According to the specification in generator-only mode, the ripple can be as large as ±7V about the DC steady state voltage.

Diode D1 in combination with capacitor C1 forms an AC rectifier that prevents high frequency ripple components from reaching the output. Note that rising edges of the input ripple waveform attempt to pull up the output capacitor, causing the LT4363 to momentarily limit the current through M2. For this reason, the current available to the output load during the ripple condition is 2.8A, less than the 4A available during normal operation. More about the ripple condition and ways to improve this circuit behavior are described in Linear Technology Journal of Analog Innovation, Volume 24, Number 1, “High Voltage Surge Stoppers Ease MILSTD- 1275D Compliance by Replacing Bulky Passive Components.”

Starting Mode

Voltage variations caused by the starter motor and cranking are described by MIL-STD-1275D starting mode—the supply voltage can drop as low as 6V before recovering to at least 16V within one second and the steady-state DC voltage within 30 seconds. The solution presented here typically functions at the 6V minimum. But it is only guaranteed to work to 8V due to component tolerances, the most significant being the loosely specified threshold voltages provided by MOSFET manufacturers.

Electromagnetic Compatibility Requirements

MIL-STD-1275D refers to another standard, MILSTD- 461, regarding electromagnetic compatibility. Typically, an EMI filter is placed at the input of MILSTD- 1275D compliant systems—while surge stoppers do not eliminate the need for filtering, their linear mode operation introduces no additional noise.

Conclusion

Linear Technology’s surge stopper products simplify MIL-STD-1275D compliance by using MOSFETs to block high voltage input surges and spikes while providing uninterrupted power to downstream circuitry. Blocking the voltage with series components avoids the blown fuses and damage that can occur when circuits attempt to shunt high energy to ground with bulky passive components.

Author

Dan Eddleman

Dan Eddleman is an analog engineer with over 15 years of experience at Linear Technology as an IC designer, the Singapore IC Design Center Manager, and an applications engineer.

He began his career at Linear Technology by designing the LTC2923 and LTC2925 Power Supply Tracking Controllers, the LTC4355 High Voltage Dual Ideal Diode-OR, and the LTC1546 Multiprotocol Transceiver. He was also a member of the team that designed the world’s first Power over Ethernet (PoE) Controller, the LTC4255. He holds two patents related to these products.

He subsequently moved to Singapore to manage Linear Technology’s Singapore IC Design Center, overseeing a team of engineers that designed products including Hot Swap controllers, overvoltage protection controllers, DC/DC switched-mode power supply controllers, power monitors, and supercapacitor chargers.

Upon returning to the Milpitas headquarters as an applications engineer, Dan created the Linduino, an Arduino-compatible hardware platform for demonstrating Linear Technology’s I2C- and SPI-based products. The Linduino provides a convenient means to distribute C firmware to customers, while also providing a simple rapid prototyping platform for Linear Technology’s customers.

Additionally, in his role as an applications engineer, he conceived of the LTC2644/LTC2645 PWM to VOUT DACs, and developed the XOR-based address translator circuit used in the LTC4316/LTC4317/LTC4318 I2C/SMBUS Address Translators. He has applied for patents related to both of these products. Dan has also developed multiple reference designs that satisfy the onerous MIL-STD-1275 28V military vehicle specification.

Dan continues to study Safe Operating Area of MOSFETs, and has created software tools and conducts training sessions within Linear Technology related to SOA. His SOAtherm model distributed with LTspice allows customers to simulate MOSFET SOA within their Hot Swap circuit simulations using thermal models that incorporate Spirito runaway.

He received an M.S. in Electrical Engineering from Stanford University and B.S. degrees in Electrical Engineering and Computer Engineering from the University of California, Davis.

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